1. Field of the Invention
This invention relates to apparatus and methods for grinding and polishing optical elements, such as lenses, prisms, windows, mirrors and similar optical systems. This invention could also be used to polish ceramics and semi-conductor surfaces.
2. Description of the Related Art
A recirculating slurry system 10 of the prior art for grinding and polishing of optical parts is shown in FIG. 1. A slurry tank 12 holds a slurry 14, which usually consists of deionized water and grinding or polishing particles of fairly uniform size, and a pump 16. The slurry 14 is pumped from the tank 12 through pipe 18 to either a delivery pipe 24 or a bypass pipe 26. If valve 20 is open, the slurry 14 is returned to the slurry tank 12, which helps keep the grinding particles of the slurry 14 in suspension by agitation.
Valve 20 is usually kept open during the grinding and polishing phases, which include grinding away the surface roughness (stock removal) along with the subsurface damaged material (usually 7 times the depth of the surface roughness (peak-to-valley) is removed). When valve 22 is opened, the slurry 14 is pumped to the grinder/polisher 30, which grinds or polishes (depending on the size of the particles used) the surface of a part (not shown). For example, a loose abrasive grinding could be performed by using a grinder with a hard lap (metal or glass). The polishing could be performed by using a polisher with a resilient material such as pitch or polyurethane. The part being ground and polished could be an optical lens or a semiconductor blank inserted in the grinder/polisher 30. During the final polishing stage, the valve 20 is usually shut or partially closed to allow the slurry to begin to settle. The slurry in the grinder/polisher 30 is returned to the slurry tank 12 via return pipe 28. It is desirable to shut valve 22 after polishing the part is complete so that slurry will circulate through bypass pipe 26 in order to avoid the settling and caking of the polishing compound.
The loose abrasive grinding used in the slurry is typically an aluminum oxide or silicon carbide with mean grit sizes between 9-30 .mu.m. Polishing slurry usually uses cerium oxide or zirconium oxide with a mean grit sizes between 1-3.5 .mu.m. For polishing slurries, the range of polishing powders on the market is usually between 0.4 to 3.7 .mu.m APS (average particle size). However, a compound with a specific particle size will have a range of different sized particles based on standard distribution curves. For example, a 2.5 .mu.m APS compound has particles ranging from 0.5 to 8.0 .mu.m in size. Although the larger particles may be a weak agglomeration of smaller particles, the surface of the polished object may be scratched if the larger particle do not break apart. In another example, a 12.5 .mu.m APS compound was tested to find that the size of the particles ranged from 7 to 25 .mu.m and some particles were as large as 40 .mu.m. Although these very large particles will most likely create scratches, these particles fortunately either settle down quickly or are too large to penetrate between the lap and the part being polished.
In order to avoid some of these problems, manufacturers perform two polishing operations: the so called "pre-polishing" process and the "final polishing" process. This is time consuming method of polishing parts because it requires each part to be removed, washed, and transferred into the final polishing machine.
FIG. 2 shows another recirculating slurry system 40 of the prior art where the pump 46 is located outside of the slurry tank 42. The slurry 44 held in the slurry tank 42 is removed via pipe 48 and valve 50. The slurry is pumped via pipe 56 to either delivery pipe 60 via valve 58 or bypass pipe 52 via valve 54. The grinder/polisher 62 receives the slurry 44 from delivery pipe 60 and returns the slurry to the slurry tank via return pipe 64.
The recirculating slurry systems shown in FIGS. 1 and 2 will pump relatively rough grit into the attached polisher. While this is advantageous during most of the polishing or grinding process, it is objectionable during the final stages of the polishing because the surface finish of substrates depends on the grit size being used. Instead of removing the partially polished part to insert into a separate "final polishing" machine, the lap is flushed with deionized water to perform a "water polishing" step. It is commonly believed that the remaining polishing particles will embed themselves in the lap material, thus exposing just the tips of the grains, which are obviously smaller than the whole slurry grains. Thus, a finer polished surface will result. However, the pH of the deionized water (pH of 7) is usually different than the optimal value for the slurry. Therefore, the slurry compound will begin to aggregate and the continued polishing of the part will create scratches. It has been demonstrated that just 15 minutes of "water polishing" deteriorates the part's surface and scratches may appear after 30 minutes. In addition, the unnecessary water added to the slurry tank will change the density of slurry and affect the polishing of subsequent parts.
The recirculating slurry systems shown in FIGS. 1 and 2 can produce an optical part with a smoothness of only 4-5 .ANG. RMS.